Vane operated solid state limit switch



United States Patent 3,009,033 11/1961 Werts inventor Everett W. WertsNormal, lll.

Appl. No. 717,334

Filed Mar. 29, 1968 Patented Dec. 22, 1970 Assignee General ElectricCompany a corporation of New York VANE OPERATED SOLID STATE LIMIT SWITCH15 Claims, 5 Drawing Figs.

1.8. CI. 307/252, 200/47, 307/305, 307/309 Int. Cl. l-l03k.17/00, H03k17/56 Field of Search 307/252, 305A, 309; 200/47 References Cited UNlTEDSTATES PATENTS OTHER REFERENCES G. E. Application Note 200.35 May 1964,Triac Control for AC Power" by E. K. Howell (page 3).

Primary Examiner-Donald D. Forrer Assistant Examiner-John ZazworskyAttorneys-David M. Schiller, Frank L. Neuhauser, Oscar B.

Waddell, Arthur E. Fournier, Jr. and Melvin M. Goldenberg ABSTRACT: Alimit switch for use with alternating current which is responsive to theposition of a magnetic vane. A Triac is employed as the currentinterrupting element of the limit switch, interrupting current whileexhibiting its normal or high impedance characteristic and conductingcurrent while exhibiting its low impedance characteristic. A magneticreed switch which is actuated by the magnetic vane, is connected inseries with the gate of the Triac, so as to control the impedancecharacteristic of the Triac.

PATENTED UEE22 mm INVENTOR. EVERETT 14/ Wsers 4 T 'OIF VE Y VANEOPERATED sou STATE LIMIT swrrcn BACKGROUND or THE INVENTION Switches ofthe magnetic vane-operated type have previously been constructed toinclude magnetic flux producing means and a magnetic reedswitch'positioned in the path of the magnetic flux. The reed switch'being' operated in response to movement of a magnetic vane relative tothe reed switch and relative to the magnetic flux, and the vane iseffective in response to arrival at a point in its path of travel toalter the magnetic flux pattern sufficiently for operating the reedswitch.

Switches of the type described above'have been widelyemployed as limitswitches wherein" the switch is operated'in response to arrival of amovable device, such as a part of a machine tool, at a particular pointin its 'path of travel to effect a control function. Wherethe controlfunction to be effected is to energize or to deenergize a motor such asfor moving the part of the machine tool, the reed switch contacts areincapa= ble of handling the 'motor current, which is generally wellabove the functional ampere current rating of the reed switch. Thereforeit has been necessary to provide in addition an electromagnetic relay,the contacts of which control the energization of the motor, and theoperating coil of which is energized through the reed switch contacts.

When a magnetic vane-operated switch is utilized as a limit switch withan electromagnetic relay as set forth above, the reed switch contactsmay open at any point in the alternating current cycle. Should thecontacts open near one of the peaks of a cycle, undesirable largeinductive voltage transients will be generated. Similarly, bouncing ofthe reed switch contacts upon closing will cause arcing which hasadeleterious effect upon the reed switch contacts. Thus, it would behighly desirable to provide a magnetic vane-operated switchfor use withalternating current which has increased power handling capabilities wasto eliminate the need for an electromagnetic relay. It would also behighly desirahleto provide a magnetic vane-operated switch whicheliminates turn-ofi' inductive volt age transients, and in which arcingupon contact bouncing when the contacts are closed is eliminated.

OBJECTS OF INVENTION tial closing of the reed switch contacts and isunaffected by subsequent contact bouncing, and in which arcing betweenthe reed switch contacts during contact bouncing is substantiallyreduced so as to greatly extend the useful life of the reed switchcontacts and therefore of the reed switch itself.

It is still another object of the invention to provide a novel andimproved vane-operated switch having power switching capabilities whichis of compact and efficient design such that it requires essentially thesame amount of space as the present vane-operated switch which includesonly amagnetic reed switch as the switching element, thus saving thespace of an electromagnetic relay which must be used with the presentvane-operated 'switch in order to provide 'power switching capabilities.i

SUMMARY THE INVENTION The foregoing objects are; accomplished inaccordance with i this inventionin one form thereof by providing avaneoperated switch in which a semiconductive bidirectional devicehaving two main current-carrying terminals andat least one additionalgate terminal as the alternating current powerswitching element. Aflux-producing member and a reed switch are positioned in thevane-operated switching housing such that a magnetic vane may be movedto positions therebetween to interrupt the flux of the flux-producingmember and thereby actuate'the reed switch contacts. The reed switchcontacts are connected to control the energization of the gate of thesemiconductive bidirectional device. Upon the first closing of the reedswitch contacts, an initiating signal is applied to the gate of the'semiconductive bidirectional device, to cause it to conduct. Uponopening of the reed switch contacts the semiconductive device willcontinue to conduct until a current zero is reached, and will then stopconducting due to the absence of a gate signal to turn it on at thebeginning ofthe next half cycle.

BRIEF DESCRIPTION OFTI-IE DRAWING FIG. I. is a cross-sectional view ofone embodiment of the solid state vane-operated switch contemplated bythis invention;

FIG. 2 is an end view of the left end portion of the solid statevane-operated switchof Fig. l illustrated with the end plate removed;

FIG. 3 is an end view of the right end portion of the solid statevane-operated switch of FIG. 1 illustrated with the end plate removed; I

' FIG. 4 is an enlarged cross-sectional view showing the semiconductivebidirectional device and the heat sink block in which it is supported,mounted on the left sidewall of the switch housing as shown in FIGS. 1,and 2; and

, FIG. 5 is a circuit diagram of the solid state vane-operated switchshown in FIGS. 14.

DESCRIPTION OF A PREFERREDEMDODIMENT Referring to the drawing, oneembodiment of the magnetic vane-operated solid state switch of thisinvention will be described. In order to protect the elements of thelimit switch they are assembled in a housing 10, and the housing iscompletely sealed to preventthe entry of iron particles and otherindustrial contaminants into the housing. A casting 12 forms the top 14,bottom 15, and two sidewalls 16 of the housing 10, as shown in FIG. 2.Since casting 12 also forms a slot 18 through which may pass a magneticvane shown by dotted lines 20 in FIG. I, it is formed of a nonmagneticmaterial such as aluminum. lnorder to close the. right and left ends ofthe housing 10 as shown in FIG. I, threaded apertures 22 as shown inFIGS. 2 and 3, are provided in the ends of the casting 12 to receivethreaded members which fasten members forming the ends of the housing tothe casting.

As shown in FIG. I, the left end of the housing is formed of a firstthicker metal plate 24 and a second thinner metal plate 26 both of whichare secured to the casting by threaded members having heads 26. Thethinnermetal plate 26 may have printed thereon a legend identifying theswitch. The right end of the housing as shown in FIG. 1 is formed by anapertured metal plate 30 and a second metal member 32 having formedtherein a threaded aperture 34. The threaded aperture 34 is provided forreceiving the end of a conduit through which ex- 1 tend the leads of thelimit switch. The plate 3.0 and the metal member 32 are secured to thecasting 12 by threaded members having heads 36. The limit switch ismounted by a metalmounting plate 37 secured to the bottom of the casting12'.

The apertures 22 in the ends of the castingJZ and similar apertures inthe bottom of the casting (not shown) are identically spaced, and anaperture is provided in the center of the switch includes a firstmagnetic flux-producing member 38 positionedon the left side of the slotlS as shown in FIG. 1, and a second magnetic flux-producing member 39positioned on the right side of the slot 18. The magnetic flux-producingmeans are shown as permanent magnets 38 and 39 which are of a generallyU-shaped configuration and include a pair of short legs connected by anelongated base section. In order to properly position the magnets 38 and39 with respect to the housing 10, positioning means are provided on theinside walls of the slot 18. Magnet 38 as shown in FIGS. 1 and 2 ispositioned by rectangularly spaced projections 40 which engage thecorners of the ends of the legs of the magnet. Similarly, magnet 39rests on four projections'42 which extend from the inside wall of theslot 18 and engage the corners of the ends of the legs of the magnet.The magnets 38 and 39 are each held in position within the housing byspring members. These spring members engage the elongated base sectionsof the magnets and the metal plates forming the ends of the housing, toapply restraining forces to the magnets. Thus, formed sheet metal spring44, the cross section of which is shown in FIG. 1, engages the elongatedbase section of magnet 38 and the metal end plate 24. Similarly, aformed sheet metal spring 46 engages the metal end plate 30 to maintainmagnet 39 in en gagement with projections 42.

It will be observed in FIG. 1, that the magnets 38 and 39 are arrangedfacing each other on opposite sides of a magnetic reed switch 48, suchthat dissimilar poles are juxtaposed. The arrangement forms a seriesmagnetic circuit with oppositely disposed air gaps spanned by themagnetic reed switch. The reed switch 48 may be of any suitableconstruction and is illustrated in FIG. 3 as comprising a glass tube 50within which are sealed a pair of elongated magnetic flexible contacts(not shown) the ends of which extend from opposite ends of the glasstube and are sealed therein. The ends of the contacts form the terminals52 and 54. The magnetic reed switch 48 is enclosed with an insulatingtube 56 and is held in position against the ends of the legs of magnet39 by one end of a spring clip 58, the other end of which is positionedbetween the elongated base of magnet 39 and sheet metal spring 46.

A limit switch comprising a reed switch and a pair of magnets positionedas thus described is the subject matter of my U.S. Pat. 3,009,033 LimitSwitches, U.S. Pat. Nov. I4, 1961, and assigned to the assignee of thepresent invention. The magnetic flux-producing member 38 which may beconsidered the operating magnet is of larger dimensions and greater polestrength than the magnetic flux-producing member 39 which may beconsidered the bias magnet.

The positions of the magnets are so determined that the reed switch ispositioned in a region wherein the magnetic field strength is very lowdue to the cancellation in the aforesaid region of the opposing leakagefluxes of the operating magnet 38 and the biasing magnet 39. Theresultant magnetic vector along the two reed contacts is thereforenormally not great enough to magnetize the reed to a degree sufficientto overcome their resilience and cause them to close. Thus, the contactsof reed switch 48 are normally open.

The reed switch contacts are actuated to their closed position when themagnetic vane is moved to a position within the slot 18 where it shuntsthe major portion of the total field contributed by the operating magnet38 and has upset the balance between the two magnetic fields within thearea occupied by the overlapping contact element of the reed switch.Since the flux component contributed by the bias magnet 39 in adirection longitudinal of the reed switch no longer has an opposing fluxcomponent to cancel its effect, the consequent magnetic vector along thereed switch results in a magnetic attraction more than sufficient toovercome the inherent resilience of the reed contacts and cause them toclose with a snap action. This closure of the contacts actually takesplace before the vane 20 has advanced to a fully effective shuntposition, that is where it extends between both opposing poles of theoperating magnet 38 and the biasing magnet 39.

According to the present invention the vane-operated solid state limitswitch has power switching capabilities, and other desirable operatingcharacteristics. While the housing 10 as shown in FIGS. l-3 is generallyof the same size as that used for earlier magnetic vane-operated limitswitches, such as the one shown in my previously mentioned U.S. Pat.3,009,033, additional elements are provided within the housing whichprovide the power switching capability. The principle element is asemiconductive bidirectional device having at least one gate terminalwhich is employed as the current interrupting element of the limitswitch.

A relatively recent addition to the growing family of semiconductivedevices broadly referred to as semiconductors, are the bidirectionaltriode P-N-P-N switches. These bidirectional triode switches areprovided with two main currenbcarrying terminals and at least one gateor trigger terminal. Current flow from the two main current-carryingterminals through the bidirectional triode switch in either directioncan be controlled by the application of a low voltage, low current pulsebetween a gate terminal and one of the load current terminals. Thesedevices have some similarity to the earlier developed silicon-controlledrectifiers. They are similar in their blocking current and voltagecharacteristics. But, unlike silicon-controlled rectifiers, they canswitch load current of either polarity. For a more complete discussionof the bidirectional triode P-N-P-N switches reference may be made to anarticle entitled: Bidirectional Triode P-N-P-N Switches, Gentry, F.E. etal. in the Proceeding of the IEEE," Volume 53, No. 4, Apr. 1965, pp.355-369. Bidirectional triode P-N-P-N switches are also discussed in abook entitled: Semiconductor Controlled Rectifiers...Principles andApplications of P-N-P-N Devices by Gentry, F.E. et al. published byPrentice-Ha1l,lnc., Englewood Cliffs, N..l., I964.

The semiconductive bidirectional device 60 shown in the drawing iscommonly called a Triac and by the International Electro TechnicalCommission standards named a bidirectional triode thyristor. While thisparticular device is shown which will hereinafter be referred to as aTriac, any other semiconductive bidirectional device which has at leastone gate terminal, may be employed. It is only necessary that thesemiconductive bidirectional device which is employed exhibit a highimpedance characteristic in the absence of a signal at its gate, andthat it exhibit a low impedance characteristic in the presence of thesignal at its gate.

The Triac 60 is shown in FIGS. 1 and 4 as having a cylindrical housing62, which .forms one of the main current-carrying tenninals, and ashaving extending from one end of the housing 62 a main current-carryingterminal 64 and a gate or trigger terminal 66.

wherein considerable heat is generated within the cylindrical housing 62of the Triac 60 when it is conducting current, it is desirable toprovide means for rapidly dissipating the heat therefrom. The meansprovided for dissipating heat is best shown in FIG. 4. A portion 68 ofthe cylindrical housing 62 of the Triac 60 is serrated and is press-fitinto a cylindrical aperture 70 in a heat sink block 72. Since thecylindrical housing 62 and therefore the heat sink block 72 is aterminal of the Triac, it is desirable to electrically insulate themfrom the switch housing, and particularly from the metal plate 24 uponwhich they are mounted, However, it is also desirable that a path of lowthermal resistance be provided between the heat sink block 72 and themetal plate 24 such that the heat from the Triac 60 may be rapidlydissipated by the switch housing 10. Such an arrangement is provided byplacing a thin sheet of insulating material 74 between the heat sinkblock 72 and the a metal plate 24. A metal screw 76 which passes throughan aperture 78 in the heat sink block 72 and is received in a I threadedaperture 80 in metal plate 24 is insulated from the heat sink block 72by an insulating washer 81 having a cylindrical portion and an annularflange portion.

Several additional circuit elements of the vane-operated solid statelimit switch of this invention are enclosed within the housing 10, andtheir position therein will be described as then are referred to indiscussing the circuit diagram shown in FIG. 5. Further, as connectionsshown in FIG. 5 are identified by a numeral, the same numeral willidentify the lead if shown in FIGS. 1 and 3. Terminal 52 of the reedswitch is connected to gate 66 of the Triac 60 by a conductor 82.Terminal 54 of the reed switch is connected through a-cuirent limitingresistor M to the main current-carrying terminal of the Triac formed byits housing 62. The connection to the housing 62 is made through aconductor 86 having a terminal pin which is received in an aperture 88in the heat sink block 72.

The main current-carrying terminal of the Triac 60 formed by thecylindrical housing 62 is connected to the circuit to be controlledthrough a conductor 90 which is provided with a terminal pin 92 receivedin. aperture 94 in the heat sink block. Main current-carrying terminal64 of the Triac is connected by a conductor 96 to one terminal of aninductor 100. The other terminal of the inductor 106 is connected to acoupling 1102 from which coupling extend toother conductors, one ofwhich conductors 104 extends from the switching housing for connectionin the circuit to be controlled. A second lead 106 extends from thecoupling to a pair of capacitors 108 and 110. As shown in FIG. 1,capacitors 108 and 1 10 and a resistor $112, one terminal which isconnected to capacitor 110, are assembled as a package 114 which isplaced between magnet 38 and the wall of the slot 18. The secondterminal of the resistor Ii 12 is connected by a lead 116 to a tab 1 18which is welded to the metal sheet M so as to provide a ground to thehousing.

The terminal of capacitor 108 is connected to heat sink block i openedposition, a signal cannot be applied to the gate 66 of the Triac 60, andtherefore even though an alternating current voltage is applied toconductors 90 and 104,-the Triac will not conduct. When magnetic vane 20is moved into slot 18 to a position whereby the contacts of the magneticreed switch 48 are closed as previously described a trigger signal isapplied to the gate 66 of Triac 60 tl'iroughcurrent-limiting resistor84. Wherein the Triac 60 is turned on within a microsecond after thecontacts of reed switch 48 initiallyclose, it is unaffected bysubsequent bouncing of the reed switch contacts. Further, while theTriac is conducting a very low voltage appears between its maincurrent-carrying terminals 62 and 64, and therefore the voltage betweenterminal 62 and gate 66 is also very low, such that the Triac providescontact protection for the reed switch contacts. This contact protectiongreatly extends the life of the reed switch as compared to its life in aconventional vane-operated limit switch wherein arcing between thecontacts is brought about by the application of higher voltages to thecontacts.

When the vane 20 subsequently moves out of the slot 18, such that thecontacts of the reed switch 48 are opened, the Triac 60 will continue toconduct until the next current zero, at which time it will stopconducting. it will not start to conduct during the next half cyclesince the open contacts of the reed switch 48 prevent the application ofa signal to the gate 66.

in order to prevent false gating of the Triac 60 due to surges ofvoltage or transient occurring on the power line, a suppressor networkis included within the vane-operated solid state limit switch. Highfrequency transient oscillations appearing in the power circuit arebypassed around the Triac 60 through the capacitor 108. In order toinsure that the transients are bypassed around'the Triac 60, theirattempt to pass through the Triac 60 is delayed by inductor 100. Anadditional series network comprising capacitor 110 and resistor 112 isconan inductive load against the stray capacitance between the Triac,heat sink combination and the metal plate 24. This balancing networkwill prevent false firing of other vaneoperated solid state limitswitches by one which is being triggered, or by a refiring due to aninductive feed back from the load being controlled.

While the vane-operated solid state limit switch of this invention hasbeen shown and described with an operating magnet 38 and a bias magnet39, it might also be constructed without the opposing bias magnet 39, onan arrangement which would provide normally closed contact operation.That is, the field of the operating magnet 38 would normally create amagnetic vector along the axis of the reed switch 48 sufficient tomaintain its contacts closeduntil the magnetic vane 20 is positioned inslot 18 so as to shield the reed switch 48 from the field of theoperating magnet 38 thereby permitting the reed switch contacts to openunder the influence of their natural resiliency. Thus, the Triac 60would nonnally conduct and would stop conducting when the magnetic vane20 entered the slot 18 to shield the reed switch 48 from the magneticfield of operating magnet 38.

While a particular embodiment of this invention has been shown anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from theinvention in its broader aspects, and therefore, it is intended that theappended claims cover all such changes and modifications that fallwithin the true spirit and scope of this invention.

lclaim:

1. A vane-operated limit switch for controlling the energization of aload from an alternating current supply comprising:

21. a housing having a slot therein, said housing being formed of anonmagnetic material;

b. at least one magnetic flux-producing device having spaced magneticpoles of opposite polarity mounted in said housing, said magneticflux-producing device being positioned on one side of said slot with itspoles facing said slot;

c. an elongated magnetic reed switch having normally open contactsclosed by mutual attraction in the presence of a magnetic flux componentof suff cient strength directed along its elongated dimension, said reedswitch being mounted in said housing on the other side of said slot in aposition where the magnetic flux from said magnetic fluxproducing devicepassing through said slot will cause its contacts to close;

d. a semiconductive bidirectional device including a cylindricalhousing, said semiconducn've bidirectional device having two maincurrent-carrying terminals and at least one additional gate terminal,said device normally exhibiting a high impedance characteristic betweensaid two main terminals and exhibiting a low impedance characteristicbetween said two main terminals in response to application of a controlsignal to said gate terminal, said two main current-carrying terminalsbeing connected to control the energization of a load from analternating current supply and said reed switch contacts being connectedto apply a control signal to said gate terminal when its contacts areclosed; and

e. a heat sink block having an aperture in which said cylindricalhousing of said semiconductive bidirectional device is received, saidheat sink block being mounted on said limit switch housing with lowthermal resistance between said limit switch housing and said heat sinkblock such that heat developed in said semiconductive bidirectionaldevice may be rapidly transmitted through said cylindrical housing andsaid heat sink block to said limit switch housing to be dissipated bysaid limit switch housing.

2. A vane-operated limit switch as defined in claim 1 wherein a resistorand said reed switch contacts are connected in series between said gateterminal and one of said main terminals, said resistor limiting the gatecurrent flowing through said reed. switch contacts.

3. A vane-operated limit switch as defined in claim 2 wherein aninductor is connected in series with the other one of said maincurrent-carrying terminals and a capacitor is connected in parallel withsaid semiconductive bidirectional device and said inductor, saidcapacitor bypassing high frequency transient oscillation around saidsemiconductive bidirectional device, and said inductor delaying thepassage of the transients through said semiconductive bidirectionaldevice to prevent false gating ofthe semiconductive bidirectional deviceby the transients.

4. A vane-operated limit switch as defined in claim 1 wherein a secondone of said magnetic flux-producing devices is positioned on the otherside of said slot with its poles facing said slot such that said firstand second magnetic flux-producing' devices form a single magneticcircuit having oppositely situated air gaps through which fluxesestablished by said devices pass in opposite directions through saidslot, said reed switch being positioned between said devices spanningsaid gaps in a region where the resultant of the fluxes is normallyinsufficient to close said contacts, said contacts being closed by themagnetic flux of said second magnetic flux-producing device, whenpassage of the magnetic vane in the slot shunts the major portion of themagnetic flux of said first magnetic flux-producing device, whereby theload will normally be deenergized and will be energized in response tothe presence of the magnetic vane in said slot.

5. The vane-operated limit switch as defined in claim 1 wherein saidcylindrical housing of said semiconductive bidirectional device is oneof its main current-carrying terminals, and said cylindrical housing andheat sink block are electrically insulated from said limit switchhousing.

6. A vane-operated limit switch as defined in claim 5 comprising inaddition a series network including a capacitor and a resistor connectedbetween said limit switch housing and the other one of said mainterminals, said series network forming a switch transient suppressor.

7. A vane-operated limit switch as defined in claim 1 wherein said limitswitch housing comprises a casting of nonmagnetic material forming theslot, the top, the bottom, and two sidewalls of said limit switchhousing through which said slot passes, one open end of said limitswitch housing being closed by a first metal plate, the other open endof said casting being substantially closed by a second metal plate, saidheat sink block being mounted on said first metal plate.

8. A vane-operated limit switch as defined in claim 1 wherein saidsemiconductive bidirectional device is a bidirectional triode thyristor.

9. A limit switch for controlling the energization of a load from asuitable electrical current supply comprising:

a. a housing having a pair of opposed walls, said housing being formedof a nonmagnetic material;

b. at least one magnetic flux-producing device having spaced magneticpoles of opposite polarity mounted in said housing, said magneticflux-producing device being positioned adjacent one of said pair ofopposed walls with said poles of said magnetic flux-producing devicefacing the other of said pair of opposed walls;

0. an elongated magnetic reed switch having normally open contactsclosed by mutual attraction in the presence of a magnetic flux componentof sufficient strength directed along the elongated dimension of saidmagnetic reed switch, said magnetic reed switch being mounted in saidhousing in spaced relation to said magnetic flux-producing device in aposition where the magnetic flux of said magnetic flux-producing devicepasses through the space between said magnetic reed switch and saidmagnetic flux-producing device causing said contacts of said magneticreed switch to close, said contacts of said magnetic reed switch toclose, sand contacts of said magnetic reed switch being opened inresponse to a decrease in the strength of the magnetic flux directedalong said elongated dimension of said magnetic reed switch;

d. a semiconductive device having two main current-carrying terminalsand at least one additional gate terminal, said device normallyexhibiting a high impedance characteristic between said two mainterminals and exhibiting a low impedance characteristic between said twomain tei minals in response to application of a control signal to saidgate terminal, said two main current-carrying terminals being connectedto control the energization of a load from a suitable electrical currentsupply and said contacts of said reed switch being connected to apply acontrol signal to said gate terminal when said contacts are closed,whereby said load will normally be energized and will be deenergized inresponse to the opening of said contacts of said reed switch; and

e. electrical transient suppressor means connected to saidsemiconductive device and said load to prevent said semiconductivedevice from being falsely fired by electrical transient generated in thecircuit.

10. A limit switch as set forth in claim 9 wherein said electricaltransient suppressor means includes a series network comprising acapacitor and a resistor connected between said housing and one of saidmain current-carrying terminals of said semiconductive device.

11. A limit switch for controlling the energization of a load from asuitable electrical current supply comprising:

a. a housing having a pair of opposed walls, said housing being formedof a nonmagnetic material; b. an elongated magnetic reed switch mountedwithin said housing, said elongated magnetic reed switch having normallyopen contacts closed by mutual attraction in the presence of a magneticflux component of sufficient strength directed along the elongateddimension of said magnetic reed switch;

0. a semiconductive device having two main current-carrying terminalsand at least one additional gate terminal, said device normallyexhibiting a high impedance charactexistic between said two mainterminals and exhibiting a low impedance characteristic between said twomain terminals in response to application of a control signal to saidgate terminal, said two main current-carrying terminals being connectedto control the energization of a load from a suitable electrical currentsupply and said contacts of said magnetic reed switch being connected toapply a control signal to said gate terminal when said contacts areclosed, whereby the load will normally be deenergized and will beenergized in response to the closing of said contacts of said magneticreed switch; and

d. electrical transient suppressor means connected to saidsemiconductive device and said load to prevent said semiconductivedevice from being falsely fired by electrical transients generated inthe circuit.

12. A limit switch as set forth in claim 11 wherein said electricaltransient suppressor means comprises an inductor connected to one ofsaid main current-carrying terminals of said semiconductive device and acapacitor connected to said semiconductive device and said inductor,said capacitor bypassing high frequency transient oscillation aroundsaid semiconductive device, and said inductor delaying the passage ofthe transients through said semiconductive device to prevent falsegating of said semiconductive device by the transients.

13. A limit switch for controlling the energization of a load from asuitable electrical current supply comprising:

a. a housing having a pair of opposed walls, said housing being formedof a nonmagnetic material;

b. a first magnetictlux-producing device having spaced magnetic poles ofopposed polarity mounted in said housing, said first magneticflux-producing device being positioned adjacent one of said pair ofopposed walls with said poles of said first magnetic flux-producingdevice facing the other of said pair of opposed walls;

a second magnetic flux-producing device having spaced magnetic poles ofopposed polarity mounted in said housing, said second magneticflux-producing device facing said one of said pair of opposed walls suchthat said first and second magnetic flux-producing devices form a singlemagnetic circuit having oppositely situated air gaps through whichmagnetic fluxes established by said first and second magneticflux-producing devices pass in opposite directions through the-spacebetween said first and second magnetic flux-producingdevices;

an elongated magnetic reed switch having normally open contacts closedby mutual attraction in the presence of a magnetic flux component ofsufiicient strength directed along the elongated dimension of saidmagnetic reed switch, said magnetic reed switch being positioned betweensaid first and second magnetic flux-producing devices spanning saidoppositely situated air gaps in a region where the resultant of themagnetic fluxes of said first and second magnetic flux-producing devicesis normally insufficient to close said contacts of said magnetic reedswitch, such contacts being closed by the magnetic flux of said secondmagnetic flux-producing device in response to shunting of the majorportion of the magnetic flux of said first magnetic flux-producingdevice;

a semiconductive device having two main current-carrying terminals andat least one additional gate terminal, said device normally exhibiting ahigh impedance characteristic between said two main terminals andexhibiting a low impedance characteristic between said two mainterminals in response to application of a control signal to said gateterminal, said two 'main current-carrying terminals being connected tocontrol the energization of a load from a suitable electrical currentelectrical current supply and said contacts of said magnetic reed switchbeing connected to apply a control signal to said gate terminal 'whensaid contacts are closed, whereby the load will normally be deenergizedand will be energized in response to the closing of said contacts ofsaid magnetic reed switch; and y f. an inductor connected to one of saidmain current-carrying terminals of said semiconductive device and acapacitor connected to said semiconductive device and said inductor,said capacitor bypassing high frequency transient oscillation aroundsaid semiconductive device, and said inductor delaying the passage ofthe transients through said semiconductive device to prevent falsegating of said semiconductive device by the transients.

14. A limit switch'as set forth in claim 13 wherein a resistor and saidcontacts of said magneticreed switch are connected between said gate;terminal and the other of said main currentcarrying terminals of saidsemiconduc'tive device, said resistor limiting the gate current flowingthrough said contacts of said magnetic reed switch.

15. A limit switch as set forth in claim 13 further comprising a seriesnetwork comprising a capacitor and a resistor connected between saidhousing and said one of said main current-carrying terminals of saidsemiconductive device.

